The objective of this work has been to understand the details of the interaction of self and foreign peptides with MHC molecules, as studied both by in vitro binding methods and by assays of T cell activation. As this project has developed, we have developed a system in which the binding of the antigenic peptide to the MHC class I molecule H-2Ld is measured and in which we can in parallel measure the interaction of the MHC/peptide complex with the T cell receptor. Current experiments have investigated the influence of the coreceptor molecule, CD8, on the MHC/TCR interaction as well as the nature of the early signaling events that are elicited by the stimulation of the T cell with peptide/MHC complexes and those containing variant peptides. Attention has been focused on a paradoxical result: the identification of a single peptide variant that complexes with the mouse MHC class I molecule H-2Ld, serves as an effective sensitizing peptide, but fails to form a complex in which we can detect binding by surface plasmon resonance. The variant peptide seems to form a complex that activates the T cells to cytolysis, lymphokine production, or now, as studied by Dr. M. Jelonek in collaboration with Dr. I. Stefanova by early phosphorylation events. Since there has been such a discrepancy in the ability of the variant peptide/H-2Ld complex to bind the TCR in vitro it is remarkable that the T cell response seems to show only minor kinetic and quantitative differences in early phosphorylation patterns. Attempts to evaluate the contribution of CD8 (produced by Australian collaborators Brendan Classon and Peter Hudson) to the binding reaction of H-2Ld/peptide complexes with the cognate TCR revealed interaction of the CD8 to H-2Ld molecules devoid of peptide. This suggested that an H-2Ld peptide motif in the amino terminal region of the CD8 protein was mediating binding to the MHC molecule through the MHC's peptide binding groove. A variety of direct binding and competition experiments have confirmed that H-2Ld can carry out this mode of binding to the mature CD8.